Nuclear magnetic resonance spectroscopy (NMR) is a powerful analytical tool due to its unique capabilities in structure determination and intermolecular interaction detection as well as its non-destructive and quantitative nature. NMR is routinely used in biomedical and synthetic applications; in particular, pharmaceutical drug discovery programs, combinatorial library analysis, and clinical analysis. With the adoption of combinatorial chemistry methods, large numbers of new compounds are being synthesized for areas such as pharmaceutical research, organic synthesis, and catalysis discovery. Large libraries of potential drug lead compounds are screened with NMR techniques in search of interactions with target protein molecules. As more libraries are produced with combinatorial reactions, the demand for high-throughput analysis increases. While NMR is well suited for the analysis of combinatorial libraries, clinical and a variety of other samples, the throughput of NMR is limiting.
Current approaches to high-throughput NMR use automatic sample changers or flow probes with robotic liquid handlers. Automatic sample changers are limited by a relatively high failure rate mainly due to the use of glass NMR tubes, which can break and also vary enough that automatic routines such as spinning the sample and finding the 2H lock can fail. Flow probe automation systems are reported to be more reliable. Typically these systems use a flow-through probe design with sample cells aligned parallel to the magnetic field. These probes use saddle-shaped Helmholtz coils with sample volumes ranging from 100–480 μL and active volumes ranging from 40–250 μL. Another approach to NMR flow probe design is the development of microcoil NMR probes. The microcoil flow probe has been used with several hyphenated techniques, such as microbore HPLC-NMR and capillary electrophoresis (CE)-NMR, and was reviewed recently. The advantage of using a microcoil probe is that less sample volume is needed (1 nL–10 μL) and the mass sensitivity (Sm, signal-to-noise ratio (S/N) per μmol of analyte) is high. This makes the system ideal for use with samples that are only available in small volumes such as natural product libraries or synthetic combinatorial compounds.
Microcoils provide another avenue to increase NMR throughput via parallel NMR detection. Multiple solenoidal microcoils can be stacked along the magnetic field axis in a single NMR probe because they are aligned perpendicular to the magnetic field and, unlike saddle-shaped Helmholtz coils, the solenoidal microcoils are small enough to fit multiple coils in one probe. Parallel analysis is common in other analytical techniques, but has only recently been explored for NMR. Various approaches have been attempted, including isolated circuits, rapid selective sample excitation, and imaging methods. Depending on the approach used, the relative signal-to-noise ratio (S/N) of each coils, compared to a single coil, in the multi-coil configuration is an important consideration. With isolated circuits, the relative S/N is not degraded except through cross-talk; however, with parallel circuits the S/N of a coil is reduced by a factor of n1/2, where n is the number of coils. We previously introduced the multi-coil multiplex NMR probe for parallel NMR analysis, which is capable of analyzing four samples at a time using chemical shift imaging or in rapid succession, with a selective excitation approach. Sample loading/unloading and data acquisition can be automated using parallel coil NMR probes to achieve truly high-throughput NMR analysis. However, the development of novel approaches to highly parallel NMR probes for higher throughput operation is an important goal.